The purpose of this quasi-experimental study was to determine the effectiveness of a computer-based training program for improving students' ability to make judgments of harmonic intonation. Twenty members of two undergraduate conducting classes at the University of Illinois participated for nine weeks in the Harmonic Intonation Training Program (HITP). Average total treatment time was nine hours per student. An equivalent matched control group was selected from 156 other undergraduate music majors who had also taken the investigator-developed Harmonic Intonation Discrimination Test (HIDT).The HITP was developed for the PLATO Interactive Music System (IMS) at the University of Illinois. The HITP consisted of a body of drill and practice exercises utilizing intervals, triads and brief three- and four-part musical passages. The exercises were played in both equal temperament and just intonation by the sixteen-voice IMS synthesizer. Three types of exercises were included: (1) Listening exercises. The student selects the mistunings of musical examples and compares those mistunings to in-tune examples in equal temperament and just intonation. (2) Judging exercises. The student makes discriminations about the location and type of intonation errors in the musical examples. (3) Tuning exercises. The student tunes intervals and chords by altering the pitch of one of the tones through keyboard input.After a nine-week treatment period an analysis of variance on experimental and control group mean posttest HIDT scores revealed a difference (p $The following additional findings were noted: (1) The results for the experimental group were significant in a practical as well as statistical sense. (2) Substantial pretest-posttest gains occurred for each of the three ability levels in the experimental group. (3) Confidence that the intonation training is transferrable to acoustical situations is warranted. (4) There was no significant difference in accuracy of judgments for sharp mistunings compared to flat mistunings. (5) Harmonic intonation discrimination skill is not highly correlated with harmonic analysis skill or ACT scores.